The present invention relates to a process for preparing polyolefin resin pre-expanded particles. Specifically, the present invention relates to a process for preparing polyolefin resin pre-expanded particles which can be suitably used as a raw material for in-mold foamed articles. In addition, the present invention relates to polyolefin resin pre-expanded particles obtained by the process and a flow-restricting device equipped with a pipe, which is used at a release port to prepare polyolefin resin pre-expanded particles.
Conventionally, there have been processes for preparing pre-expanded polyolefin resin particles which comprises steps of: dispersing polyolefin resin particles into an aqueous dispersion medium in an autoclave; heating the mixture; impregnating blowing agent thereinto; and releasing the particles into a low-pressure vessel to obtain polyolefin resin pre-expanded particles of a desired shape such as sphere, cylinder, ellipse or cube (Japanese Examined Patent Publication No. 5784/1995). As blowing agents, volatile blowing agents such as organic volatile blowing agents, inorganic gas and water are used (International Publication 97/38048 Pamphlet (1997)). As organic volatile blowing agents, propane, butane, pentane, trichlorofluoromethane and dichlorodifluoromethane are known (Japanese Unexamined Patent Publication No.77174/1977).
However, in case of preparing pre-expanded particles having expansion ratio of at most 15 times according to the above process, impregnation unevenness of blowing agents to resin particles are increased because it is necessary to reduce the amount of the blowing agent. As a result, fluctuation in expansion ratio calculated by the following formula (I) becomes as high as at least 20%, and pre-expanded particles having excellent properties cannot be obtained. Fluctuation in expansion ratio is calculated by the following formula (I):
Fluctuation in expansion ratios ("sgr"/Xp)(%)=("sgr"mKav)xc3x97100xe2x80x83xe2x80x83(I)
In the formula, Kav represents average expansion ratio calculated by the equation Kav=xcexa3{Kixc3x97Wi} based on weight percentage Wi and expansion ratio Ki of the remaining foamed particles after screening through JIS Z 8801 standard screens (eight screens: 3.5, 4, 5, 6, 7, 8, 9, and 10 mesh); and "sgr"m represents standard deviation calculated by the equation "sgr"m=xcexa3{Wixc3x97(Kavxe2x88x92Ki)2} based on weight percentage Wi and expansion ratio Ki of the remaining foamed particles after screening through JIS Z 8801 standard screens (eight screens: 3.5, 4, 5, 6, 7, 8, 9, and 10 mesh).
When fluctuation in expansion ratios of pre-expanded particles is increased, pre-expanded particles having desired expansion ratio are difficult to obtain in the process for preparing pre-expanded particles. Accordingly, it becomes difficult to control expansion ratios, with yield being lowered at the same time. In addition, there is a problem that fluctuation in weights of in-mold foamed articles obtained by using pre-expanded particles as a raw material is increased and it becomes difficult to produce in-mold foamed articles having excellent properties, and defective products are increased.
Furthermore, the above process has a defect that costs are increased since volatile blowing agent is used. In addition, the volatile blowing agent should not be used from the viewpoint of preventing progress of global warming and growth of ozone hole.
Japanese Unexamined Patent Publication 4738/1986 discloses a process for preparing pre-expanded particles using polypropylene resin particles containing volatile blowing agent or inorganic gas, wherein the polypropylene resin particles contains 0.05 to 2% by weight of an inorganic substance such as aluminum hydroxide or calcium carbonate. When volatile blowing agent is used, there are cost and environmental problems as mentioned above. In addition, inorganic gas other than nitrogen and air, such as carbon dioxide should not be used because such gas contributes to greenhouse effect as well. Further, even if inorganic gas other than carbon dioxide, i.e., nitrogen or air is used, there have been problems that foamed articles are difficult to obtain and that even if foamed articles are obtained, they only have an apparent expansion ratio of at most 2 to 9 times because nitrogen or air has low permeability to polypropylene resin particles.
For these reasons, it is recently sought to develop processes which enable to produce polyolefin resin pre-expanded particles having desirable properties without using volatile blowing agent which has been considered to be necessary.
A process for preparing polyolefin resin pre-expanded particles without using volatile blowing agent is disclosed in Japanese Examined Publication No. 2183/1974. The process comprises steps of dispersing, into a dispersion medium, crystalline polyolefin polymer particles containing 10 to 70% by weight of a filler based on a polymer; keeping the obtained dispersed solution within a high pressure area in which the pressure is at least saturated vapor pressure of the obtained dispersed solution under a temperature condition of at most the melting point of the crystalline polyolefin polymer particles and such that crystallization of crystalline polyolefin polymer particles proceeds, thereby impregnating the dispersion medium into the polymer particles by means of volumetric shrinkage caused by the crystallization of the crystalline polyolefin polymer; and releasing the dispersed solution containing the obtained foamable crystalline polyolefin polymer particles from the high pressure area to a low pressure area, the pressure being lower than that of saturated vapor pressure of the dispersed solution and the temperature being at least the melting point of the crystalline polyolefin polymer particles.
However, though this process requires for a crystalline polyolefin polymer to contain 10 to 70% by weight of a filler based on the polymer to prepare pre-expanded particles, a molded article obtained by expanding such pre-expanded particles has a large amount of fillers, and thus deterioration of properties such as flexibility or cushioning property is unavoidable. Accordingly, such pre-expanded particles cannot be used for high level purposes and have a problem that they prevent particles from fusing with each other at the time of in-mold molding.
Further, Japanese Unexamined Patent Publication No. 22144/1985 suggests another process in which no volatile blowing agent is used. The process comprises steps of pressurizing a dispersed solution in which resin particles of a propylene-ethylene random copolymer having ethylene content of 1 to 12% by weight are dispersed by using inorganic gas such as nitrogen so that the inner pressure of the closed vessel reaches at least 5 kg/cm2-G; and then keeping the dispersed solution at temperature of from at least the melting point of resin particles to at most 25xc2x0 C. higher than the melting point to obtain pre-expanded particles having expansion ratio of at least 5 times.
However, when pre-expanded particles having high expansion ratio are to be prepared according to the above process, it is necessary that the ethylene content of the propylene-ethylene random copolymer resin particles is at least 4% by weight and that keeping temperature is at least 160xc2x0 C. or keeping time is 10 hours. In addition, when additive resins such as polyethylene, an ethylene-vinyl acetate copolymer, Surlyn and polystyrene are used, pre-expanded particles to be obtained have large fluctuation in expansion ratio. In addition, mechanical strength and heat resistance inherently required for such polypropylene resin foamed articles are insufficient as in the case of the above pre-expanded particles obtained under the condition of ethylene content of at least 4% by weight. Besides, preparation under such keeping temperature and keeping time easily causes fusion of resin particles, which means that such preparation is non-productive and uneconomical.
On the other hand, Japanese Unexamined Patent Publication No. 176077/1998 discloses a process for preparing pre-expanded particles without using blowing agents, i.e., combustible gas such as butane and carbon dioxide gas, which contribute to the progress of global warming. That is, the process for preparing pre-expanded particles comprises steps of heating a polyolefin resin having 100 parts by weight of a polyolefin resin and 0.05 to 20 parts by weight of a hydrophilic polymer to prepare water-containing resin particles; releasing the water-containing resin particles into a lower pressure atmosphere. This is an innovative preparation process which enables to keep the fluctuation in expansion ratios to at most 15% even in case of pre-expanded particles having expansion ratio of as low as at most 15 times. However, further decreased fluctuation in expansion ratios has been desired in order to improve yield of pre-expanded particles in production steps and to prepare molded articles having less fluctuation in weights with excellent properties.
As mentioned above, it is still very difficult at present to obtain pre-expanded particles which can provide molded articles having excellent mechanical properties, heat resistance, water resistance, flexibility and cushioning property without using conventional volatile blowing agents, i.e., blowing agents such as carbon dioxide which are not preferable in view of promoting global warming and causing other environmental problems.
Furthermore, as to the release of resin particles, they are released through an orifice (a slit or a hole) once, which is provided on an orifice or nozzle flow-restricting device located at the release port (for example, Japanese Unexamined Patent Publication No. 197027/1983).
Examples of such orifice or nozzle flow-restricting devices located at the release port include a flow-restricting device having 1 to 10 slits which are 4 to 20 mm wide, 4 to 20 mm high and 5 to 200 mm deep at the narrowest part, or a flow-restricting device having 1 to 10 holes which has an area of 10 to 400 m2 and 5 to 200 mm deep at the narrowest part.
However, when lowly pre-expanded particles having an expansion ratio of 2 to 15 times are to be prepared by using the above flow-restricting device, fluctuation in expansion ratios becomes as high as at least 20%, making it impossible to prepare pre-expanded particles with excellent properties.
An object of the present invention is to solve the above problems in prior arts and to provide a process for preparing polyolefin resin pre-expanded particles having further reduced fluctuation in expansion ratios.
Another object of the present invention is to provide a process for preparing polyolefin resin pre-expanded particles having excellent low expansion ratio or excellent high expansion ratio and low fluctuation in expansion ratios without using volatile blowing agent or carbon dioxide gas by using water in polyolefin resin particles as effective blowing agent, and pre-expanded particles obtained by the process.
The present invention enables to prepare polyolefin resin pre-expanded particles having further reduced fluctuation in expansion ratios by the process comprising steps of: dispersing polyolefin resin particles into an aqueous dispersion medium in an autoclave; heating the resin particles to temperature of at least the softening point of the polyolefin resin and pressurizing the same; releasing the resin particles from the autoclave through a release port into an atmosphere of lower pressure than the inner pressure of the autoclave, thereby pre-expanding the resin particles, wherein at the releasing step, pre-expanded particles are collided with a collision board or a container wall. In addition, it has been confirmed that the process has an effect to improve expansion ratio as compared with the case without collision. It has also been found that the amount of the blowing agent to be used can be reduced and there is an effect of achieving energy saving.
In addition, preparation of polyolefin resin pre-expanded particles having a desired expansion ratio and properties has been completed without using volatile blowing agents by vigorously colliding resin particles released from the released port with a collision board or container wall to evaporate water in the resin particles instantaneously.
Furthermore, it has been found that pre-expanded particles having an expansion ratio of at least 10% can be obtained by changing the shape of the flow-restricting device used for the release port to a particular shape different from conventional board, and the present invention has been completed.
That is, the present invention relates to a process for preparing polyolefin resin pre-expanded particles, comprising steps of: dispersing polyolefin resin particles into an aqueous dispersion medium in an autoclave; heating the resin particles to temperature of at least the softening point of the polyolefin resin and pressurizing the same; releasing the resin particles from the autoclave through a release port into an atmosphere of lower pressure than the inner pressure of the autoclave, thereby pre-expanding the resin particles, wherein at the releasing step, pre-expanded particles are collided with a collision board or a container wall located at a distance of farther than 5 mm from the release port.
It is preferable that the pre-expanded particles are contacted with gas of at least 60xc2x0 C. and collided with a collision board or a container wall located at a distance of farther than 5 mm from the release port.
It is preferable that the pressurization is carried out by introducing inorganic gas selected from the group consisting of nitrogen gas, air and gas comprising those as a main component.
It is preferable that the pressurization is carried out with a pressure of 0.6 to 7.5 MPa.
It is preferable that the pre-expanded particles are collided with a collision board or a container wall at a collision angle of 5 to 85 degrees.
It is preferable that the polyolefin resin particles are released through a flow-restricting device equipped with a pipe.
It is preferable that the gas of at least 60xc2x0 C. is selected from the group consisting of vapor, a mixture of vapor and air, heated air, mist-containing air, heated vapor and superheated vapor.
It is preferable that the gas of at least 60xc2x0 C. is saturated vapor.
It is preferable that a pipe having an open area of 1.3 times larger than the open area of an orifice is integrally mounted on the flow-restricting device equipped with a pipe at the release side of an orifice board.
It is preferable that the flow-restricting device is equipped with at least one pipe which has H of at least 0.6 mm and L of at least 5 mm, H representing a width or a minor axis of a front face of the pipe and L representing a pipe length.
It is preferable that H is 3 to 25 mm and L is 5 to 300 mm.
It is preferable that the polyolefin resin particles are polypropylene resin particles.
It is preferable that the polypropylene resin particles comprise 100 parts by weight of a polypropylene resin and 0.01 to 20 parts by weight of a hydrophilic polymer.
It is preferable that the polyolefin resin particles contain 0.001 to 5 parts by weight of an inorganic filler.
It is preferable that the inorganic filler is talc.
The present invention also relates to polyolefin resin pre-expanded particles obtained by the process.